Oscillation generator



oscILLATIoN GENERATOR Filed sept. 1l. 1926 Patented Dec. 15, 1931 UNITEDSTATES PATENT OFFICE ROY E. CORAM, 0F NEWARK, NEW JERSEY, ASSIGNOR TOWESTERN ELECTRIC COMPANY,

INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK OSCILLATIONGENERATOR Application led September 11, 1926'. Serial No. 134,777.

rllhis invention relates to the generation of sustained oscillations,and aims to avoid undesired variation of the frequency of theoscillations with temperature.

As an example of applications of the invention, there is shown anddescribed herein a specific form ofelectric space discharge oscillatorembodying the invention. Temperature changes may cause undesiredvariation of the oscillation frequency of an electric space dischargeoscillator by changing the reactance of a tuning element in the tunedcircuit customarily employed for setting the frequency of theoscillator. i For example, condensers commonly used for tuning suchfrequency determining circuits Vary in capacity when they are subjectedto tempera.- ture change. It has been found that, in the range oftemperatures at which they are ordinarily operated, their temperaturecoeflicient of capacity is smaller for condensers of large capacity thanfor condensers of small capacity. iMoreover, for reasons indicatedhereinafter', as the tuning inductance in the frequency determiningcircuit of an electric space discharge oscillator is decreased and thetuning capacity in the frequency determining circuit is correspondinglyincreased to maintain the frequency of oscillation constant, a limitingvalue of inductance is reached, which is the lowest value that theinductance can have if the system is to generate sustained oscillationsof the constant frequency. In accordance with the invention, the tuninginductance is given a value in the neighborhood of this critical valueto enable the tuning capacity in the frequency determining circuit to begiven approximately its maximum possible value, so that the variation ofthe frequency of oscillation due to the temperature changes to which theCapacity is subjefted will be small.

Other objects and features of the invention will be apparent from thefollowing description and claims.

The single gure of the drawing is a circuit diagram of an electric spacedischarge oscillator embodying a form of the invention.

In the figure is shown an electric space discharge device comprising avessel 1 enclosingV a cathode 2, an anode 3, and a control electrode orgrid 4. Heating. current is su lied to the cathode 2 by the source of E.5; A source of E. M. F. 6, having its negative Y terminal connected tothe cathode 2Y and its 55 positive terminal connected to the anode 3through a choke coil 7, supplies'space current to the discharge device.The control electrode or grid 4 is'negatively polarized by a source ofsteady E. M. F. 8 included in the I6o circuit connecting the cathode andgrid of the discharge device, which also includes a coupling coil 9; l

The output circuit of the device is provided with twooscillating currentpaths in addition tothe space current path. The first oscillation pathextends from the anode 3 through the variable resistance 11, tunedcircuit 12 and blocking condenser 10 of large capacity to the cathode 2.The circuit 12 "I0 serves to Vdetermine the frequency of theoscillations generated. The constants of circuit 12 may be varied byadjusting eitherthe inductance 14 or capacity 15, as indicated by thearrows, or by adjusting both of these ele- 75 ments. The inductanceAcoil includeduin the tuned circuit 12 is inductively associated withthe coil 9. By means of this coupling oscillating current, of aVfrequency determined by the constants of the circuit 12 andequal to theresonance frequency of that circuit is fed back to the input circuit tocontrol the production of oscillations.

The second oscillating current output path maybe traced from the anode 3through the i* resistance 13, blocking condenser 10 of large capacity,and load impedance 22, to cathode 2.

The resistance 13 is preferably suiiiciently high to prevent changes inthe load impedy0 ance 22 from unduly affecting the impedance of theoscillating path including it, as seen from the tube 1, and therebytending to cause undesired change in the frequency of the oscillator. un

In' order to produce oscillating current of sinusoidal Waveform of thefrequency determined by the constants of the circuit 12, the resistance11 should preferably be sufficiently large to limit the amplitude of the1W generated Waves t-o such a value that the device 1 operates on themost nearly linear portion of its characteristic curve representing therelation between input voltage and output current. The resistance 11 canalso be used to prevent the oscillator from changing its output voltageand consequently the energy delivered to the load 22, due to changesproduced in the oscillator frequency by manipulation of capacity l5 orinductance 14. However, the resistance 11 should not be unduly great,since, as will be made apparent hereinafter, the smaller is resistance11 the smaller may be inductance 14 and the greater may be capacity 15and, consequently, the smaller -will be the changes in the oscillatorfrequency, caused by temperature variations of capacity 15.

The circuit arrangement as described above is similar to the circuitarrangements disclosed in U. S. patent to Terry, 1,573,948, grantedFebruary 23, 1926.

The inductance 14 is, preferablyA of a type in which the temperaturecoefficient of inductance is small, as Vfor example, an air coretype.

Preferably, condensers which have low temperature coefficients ofcapacity and at the same time afford considerable magnitudes of capacityin compact and inexpensive form, 'as for example, the so-called drystack mica condensers constructed of alternate sheets of metal foil andmica insulation clamped together, are used in obtaining the capacity 15.Tests have shown the temperature coefficient lof the capacity of suchcondensers to be an vinverse function of capacity, in the temperaturerange in which they are ordinarily employed. Therefore, it is desirableto use condensers having relatively large values of capacity, to givethe oscillator stability as regards the frequency of the oscillationsgenerated. For example, according to tests of the temperaturecoeflicient of capacity of commercial dry stack mica condensers, the useof capacity values greater than 0.1 mf., with a maximum temperaturecoefficient of approximately .0022 per degree Fahrenheit in atemperature range from approximately 50 to 100 F., would result in amaximum Nariation of about .3 cycle per degree at 30,000 cycles, or amaximum oscillator temperature coefficient of frequency of about .0011%if .all other parts of the circuit were perfect. As another example,according to tests of temperature coetlicients of commercial dry stackmica condensers, if a condenser having a'capacity variable from .01 to.06 mf. furnish nearly all of the capacity in the frequency determiningcircuit of an oscillator, the oscillator temperature coethcient offrequency will be approximately .0038% or about 1.1

.cycles per degree Fahrenheit at 30,000 cycles, again assuming all otherparts of the circuit to be perfect. For a dry stack mica condenser ofgiven capacity, the temperature coeficient of capacity decreases as thearea of each plate is decreased, but considerations of economy limit theextent to which it is commercially practicable to reduce the plate area.Moreover, it is in the interest of economy in manufacture to use platesof the same size for condensers of different capacities, as far aspracticable.

Although is it desirable to use condensers having as large capacity aspossible for tuning the frequency determining circuits of oscillators,there is a limit of capacity which cannotbe exceeded in an oscillatoremploying a given electric space discharge device, because the resonancefrequency of the tuned circuit varies as the reciprocal of the squareroot of the product of the inductance and the capacity of the tunedcircuit and, as noted above, as the tuning inductance is decreased andthe tuning capacity is correspondingly increased to maintain thefrequency of oscillation constant, a limiting value of inductance isreached, below which the system will not oscillate at the givenfrequency. (When the inductance has any value lower than this criticalvalue, the system will produce no sustained oscillations unless thecapacity in the tuned vcircuit be decreased, in which case the frequencyof the oscillations generated will increase.) l

It is clear that in the oscillation generating system shown, forcontinuous operation the voltage impressed on the grid must never beless than 1 ,a times the voltage developed in the plate circuit.l Arigorous calculation of the circuit conditions would include the energyconsumed by the resistance RL of the resistor 13 and the load 22, but inthe circuit under consideration this resistance RL is Vpreferably large`compared with the sum of the internal plate to filament resistance ofthe tube 1 and the resistance RF of resistor 11, so that, since theomission will greatly simplify the work, the resistance RL will beconsidered as infinite in the discussion to follow.

Then, letting tube, 1:2 Tr f where ,t is :frequency of oscillations fgenerated,

' coL Q f r=etfective resistance of coil 14 at fre- Yeo K: voltagetransformation ratio of coil '14, the following equations may bewritten:

ILK

RF may be reduced to 0 as a limit .so we may write our absolute limitingcondition as Equating and and dividing by a 152K or E:

L or (solvmg for L Ra (5) i' pK- l but i r'.'

and

(7) ring substituting in (5) I L Ro (8) wf ,1K-1

wZLQ

R0 (9) L :weoK-o or replacing Q by 91% l Rar Y (10) L- w @Fl-1) theratio of the capacity of the frequency de-A termining circuit to theinductance of that circuit larger than some critical value and `stillhave the oscillator produce oscillationsof a given frequency, is thatthe effective impedance of the frequency determining circuit is notVinfinite but is a function of the distributed resistance in theinductance and in the capacity. Y i

The minimum value that the inductance of the frequency determiningcircuit of an oscillator' may have for a given frequency may bedetermined experimentally, if desired,'by

decreasing'the value of the inductance, while maintaining constantfrequency, until the oscillator just ceases to generate sustainedoscillations.

In general, the value of resistancey 11 is preferablyl zero when thefrequency of the oscillator is approximately at a critical value, thatis, when the frequency of the oscillator is approximately at avaluebelow which the oscillator will not generateoscillations unless theinductance of thefrequency determining circuit be increased. If theoscillator' frequency be then materially increased from that value bydecreasing the capacity of the fre-' quency determiningcircuit, it maybe desirable to increase the value of'resistance 11 to prevent undueincrease in the oscillator out-V put E. M, F. If the desired increase inoscillator frequencybe obtained by decreasing the inductance in thefrequency determining Acircuit andY also decreasing the lcapacity insuch manner that L will always have `the value shown in Equation 9,above, then the value of resistance 1l can ordinarily remain Zero@through the range of variation of the inductance in which the quantityQ, defined above, remains sensibly constant. f f

` Although the maximum advantage obtainY able by use of the inventioncan be realized in any given oscillator at only one frequency for agiven value of inductance Vin the frequency determining circuit, theapplicability of the invention to oscillators and its advantages in thatconnection are not restrictedl to single frequency oscillators. When itis desired that an oscillator operate at different frequencies, if thevariation of frequency with temperature is to be as small as possiblefor each frequency then the changes in frequency i are preferablyobtained by varying the inductance of the frequency determining circuit,so that for each frequency the inductance may have the minimum valueconsistent with generation of sustained oscillations of thatfrequencyand consequently the capacity of the frequencyv determining circuit maybe correspondingly changed so as to have at each frequency the largestvalue consistent with the generation of sustained oscillations at thatfrequency. If it be not feasible to vary the inductance, then theinductance should preferably have the lowest value consistent with thegeneration of sustained oscillations vofthe lowest frequency which theoscillator Way of example,

is required to deliver, the desired frequency changes being thenobtained by variation of -the capacity of the frequency determiningcircuit. Y

In the latter case, it should be borne in mind that for every value ofthe capacity in the frequency determining circuit, the capacity, or tosay the least, that portion of it which changes its value withtemperature `change should preferably have the largest percentagepracticable concentrated in a singlexcondenser. It may frequently beadvisable, as for example, for the purpose of adj ustment of'the totalvalue of capacity in the frequency determining circuit, to employ arelatively small air or mica condenser, or both, in conjunction With alarger (preferably much larger) mica condenser. If, then, the capacitysupplied in the circuit by the larger mica condenser be sufficientlylarge compared to the capacity supplied in the circuit by the smallermica. condenser, the capacity variations of the smaller mica condenserwith temperature changes Will be suoli a small percentage of the totalcapacity 1n circuit that the smaller mica condenser can not undulyaffect the temperature coefficient of frequency of the oscillator eventhough the mcapacity'of the smaller mica condenser is so low that thetemperature coeicient of capacity of that condenser is considerablygreater than the temperature coeiiicient of capacity of the larger micacondenser. Moreover, since the capacity supplied in the circuit by thelarger mica condenser will under these conditions be approximately thewhole of the capacity in circuit, this condenser will have approximatelythe same temperature coefficient of capacity that it vvould have Wereits capacity equal to the slightly larger value of the total capacity inthe circuit. Where the variable capacity in the frequency determiningcircuit is to afford any desired capacity between, say, 0 and about 0.1mf., it may, by consist of one bank of dry stack mica condensersaffording a capacity variable from .01 to 0.1 mf., in parallel With anair dielectric condenser variable from zero to about 1200 mmf. and inparallel With a second bank of dry stack mica condensers affording acapacity variable from .001 to .01 Vmf., the first bank of micacondensers consistgiven frequency, utilizing a frequency determiningcircuit comprising a variable inductance and capacitance in resonantrelation, and amplifying means for regeneratively supplying oscillationsto said circuit and receiving oscillations therefrom, which consists insetting the inductance at a given value, adjusting the capacitance tothe largest value at which oscillations persist, varying the inductancein such direction as to cause the oscillation frequency to more nearlyapproximate the desired value, readjusting the capacitance as before tothe corresponding new limiting frequency, and making repeatedreadjustments of inductance and capacitance in the above order until thedesired oscillation frequency is achieved.

2. The method recited in cla-im 1, in Which the degree of adjustment ofinductance and capacitance is made progressively less as the desiredfrequency is approximated in order to more nearly achieve a finalcoincidence of desired frequency and maximum capacitance for sustainedoscillations.

3. The method recited in claim 1, utilizing in the frequency determiningcircuit a variable inductance and a variable capacitance element havinga capacitance-temperture characteristic which varies inversely as thecapacitance.

4. The method of generating a wave of a given frequency, and, for suchfrequency, having a minimum frequency-temperature coelicient, utilizinga frequency determining circuit comprising .a variable inductance and avariable capacitance element having a capacitance-temperaturecharacteristic which varies inversely as the capacitance, and amplifyingmeans for regeneratively supplying oscillations to said circuit'a'ndreceiving oscillations therefrom, Which consists in initially adjustingthe resonant frequency of said circuit according to the oscillationfrequency desired and subsequently simultaneously decreasing theinductance and increasing the capacitance While keeping the resonantcondition, hence the oscillation frequency, constant until a furthervariation of values tends to prevent the system from sustainingoscillations.

5. The method of generating a Wave of a given frequency, and, for suchfrequency, having a minimum frequency-temperature coeiiicient, utilizinga'frequency determining circuit comprising an inductance coil having anair core and a capacitance element made up of alternate sheets of metalfoil and mica insulation, and amplifying means for regenerativelysupplying oscillations to said circuit and receiving oscillationstherefrom, which my name this 9th day of September, A. D.,

ROY E. CORAM.

